Non-visible areas of materials, such as the interiors of components, welds and composite materials can be analysed using ultrasonic testing. This type of non-destructive testing utilises the reflection of sound waves to detect faults and features which would otherwise be very difficult to detect without destroying the component in the process. Ultrasonic testing is a common technique in the aerospace sector to test the integrity of materials at manufacture and during service.
Phased array ultrasonic testing scanners generally include a transducer having a plurality of transducer elements. The array of transducer elements may be selectively energised to launch a number of individual wave fronts into a test material. The wave fronts constructively and destructively combine within the material, resulting in a primary wave front which travels through the material and reflects off cracks, discontinuities or the like therein. Such cracks, discontinuities and the like will be herein referred to as “target features”. The primary wave front can be considered to define a beam. The beam can be dynamically steered across a scanning envelope by firing groups of transducer elements in a particular sequence. The sequence and related parameters (e.g. voltage) are sometimes referred to as a “focal law”.
The signal response from primary wave front at a plurality of contiguous beam angles can be combined by the scanner to create a two dimensional “S-Scan” image corresponding to the scanning envelope. This type of scanner is known in the art as “sectorial” or “angular” scanning. The transducer of such a scanner has a central transmission axis and the amount by which the envelope extends from the central axis may be represented in degrees e.g. +50°>−50°, or +70°>+50°, although the envelope may also be expressed with reference to a plane, such as a central plane, or an axis of the test object. When the scanning envelope extends in both the positive and negative directions it is known as a “lateral” scan.
While it may be advantageous to increase the size of the scanning envelope, doing so can be problematic because the magnitude of a primary wave front created by the energised transducer elements may vary across the envelope. Thus, the response signal strength from a target feature at one angle from the central axis may differ to that from an identical target feature at another angle from the central axis. This may result in a loss of signal amplitude from a range of values when visualising signal response, which may relay to a user of the scanner that a target feature at, say, +50° is smaller than a target feature at, say, +20°, when in fact both target features are of the same size and equidistant from the transducer.
Some ultrasonic testing scanners can compensate for the above-mentioned problem by providing sensitivity calibration functionality. One example is the Omniscan MX scanner marketed by Olympus. The sensitivity calibration process generally involves moving the transducer along a “calibration standard” block which is provided with a cross drilled hole and using the signal response from the cross drilled hole to calibrate the control signals to the transducer elements. However, even following such sensitivity calibration, the signal response across a wide scanning envelope may be more irregular than is desirable. Following this, a separate “reference standard” block is scanned using the transducer to set the calibration gain of the ultrasonic testing scanner such that the gain is at an appropriate level to search for a target feature.